The present invention relates to a process for adhering polyurethane elastomer to metal to obtain an adhered composite article having excellent water resistance and heat resistance.
As a polyurethane elastomer has superior physical properties to other synthetic elastomers and rubbers, it is used as an engineering plastic material. Recently while attracting attention to the properties of polyurethane elastomer, a technology to improve the durability of the metal by coating the surface of a metal article with polyurethane elastomer has been developed. As an adhesive for adhering polyurethane elastomer to the metal surface, phenolic resins or epoxy resins have hitherto been known. Such resins have been utilized in adhering high temperature cure castable polyurethane elastomer to the metal and the physical properties and adhesive property of the thus prepared polyurethane elastomer is improved by heating the cast elastomer for a long period from a few hours to one day at a temperature of 100.degree. to 150.degree. C.
In the case of adhering polyurethane elastomer to metal, it is uneconomic from the energy view point to maintain the cast polyurethane elastomer for a long period at such a high temperature, and it is difficult to carry out the casting at a high temperature. Although a process for casting polyurethane elastomer at a lower temperature for a shorter period has been proposed, in the case where adhesion of metal and polyurethane elastomer is carried out at a relatively low temperature of 20.degree. to 80.degree. C. by casting while utilizing the above-mentioned phenolic resin or epoxy resin, the thus obtained adhered composite article shows the peel strength of only 1 to 2 kg/cm.
As a result of studying methods for obtaining an adhered article in which metal and polyurethane elastomer are strongly adhered together even when polyurethane elastomer is cast at a low temperature, the present inventors have found that by using the two specified adhesive compositions, polyurethane elastomer is adhered to metal with a sufficiently large adhesive strength and adhered composite article thus obtained has excellent water resistance and heat resistance even if the polyurethane elastomer is cast at a low temperature.
Namely, the present invention relates to a process for adhering polyurethane elastomer and a metal which comprises coating the surface of the metal with a composition containing a polyepoxy compound and a polyamine compound, further coating the thus coated surface with a composition containing an isocyanate compound having an isocyanate group concentration of 15 to 50% by weight and casting the polyurethane elastomer onto the thus double coated surface of the metal followed by hardening the whole system.
The present invention will be further explained more in detail as follows:
In the process of the present invention, two kinds of adhesive compositions are used. The composition containing both the polyepoxy compound and the polyamine compound is referred to as the primary primer, and the composition containing the isocyanate compound is referred to as the secondary primer.
The polyepoxy compound used as the major component of the primary primer is preferably a compound having terminal epoxy groups which is prepared by adding epichlorohydrin to bisphenol A and has an average molecular weight of 300 to 4,000; a polyepoxy compound prepared by reacting epichlorohydrin with polyphenolic resin obtained by condensing a phenolic compound such as phenol, cresol, resorcinol, and the like with an aldehyde such as formaldehyde, glyoxal, acrolein, and the like. It is possible to use commercialized polyepoxy compounds, for instance, Epikote.RTM.-828, -1001, -1004, -1009, -152 and -154 (manufactured by Yuka-Shell Epoxy Co.) or Araldite.RTM. EPN-1138 and -1139, and ECN-1235, -1273 and -1280 (manufactured by Ciba-Geigy Ltd.) and the like. These polyepoxy compounds are used singly or as mixtures thereof. In the case where a highly heat resistant product is required, epoxy compounds prepared by reacting polyphenolic resin and epichlorohydrin are preferable.
As the polyamine compound used as a hardening agent in the primary primer various kinds of hardening agents, including commercially known hardening agents are used.
For instance, a polyamine such as diethylenetriamine, tetraethylenepentamine, hexamethylenediamine, propylenediamine, trimethylhexamethylenediamine, isophoronediamine, menthenediamine, xylylenediamine and the like; a reaction product of such polyamine with an epoxy compound such as 1,4-butanediol diglycidylether, glycerine triglycidylether, bisphenol-A diglycidylether and the like; a compound having terminal amino groups which is synthesized from a dimer acid and a polyamine such as ethylenediamine, propylenediamine, hexamethylenediamine, diethylenetriamine, xylylenediamine and the like, which is commercially available as Versamide.RTM.-115, -125 and -140 (product of Japan Henkel Co.); a compound obtained by the Mannich reaction of formaldehyde and a phenolic compound such as phenol and cresol and a secondary amine such as dimethylamine and diethylamine; or a polyamine obtained by the reaction of an aromatic amine such as aniline and toluidine and an aldehyde such as formaldehyde can be used. In the case where a highly water resistant and heat resistant product are required, it is preferable to use an aromatic compound such as xylylenediamine, its epoxidized product and its amidation product.
It is more preferable to prepare the primary primer by combining the epoxy compound obtained by reacting epichlorohydrin with polyphenolic resin as the major component and xylylenediamine or a compound having terminal amino groups obtained by reacting a part of xylylenediamine with an epoxy compound as the hardening agent.
For further improving the adhesive properties and water resistance of the adhered composite article, it is preferable to add a silane coupling agent to the primary primer.
As the silane coupling agent, .gamma.-glcidoxypropyltrimethoxysilane, .gamma.-aminopropyltriethoxysilane, .gamma.-mercaptopropyltrimethoxysilane and the like or the reaction product of .gamma.-glycidoxypropyltrimethoxysilane with .gamma.-aminopropyltriethoxysilane or .gamma.-mercaptopropyltrimethoxysilane is preferable.
In addition, the following additives may be added further to the primary primer. For example, an extender pigment such as talc, clay, calcium carbonate, calcium sulfate, alumina, silica, calcium silicate and the like; a colour pigment such as titanium oxide, carbon black, iron oxide, lead oxide, ultramarine blue, phthalocyanine blue and the like; an anti-corrosive pigment such as lead silicate, calcium plumbate, lead chromate and the like and an antisetting agent such as colloidal silica, asbestos, hydrogenated castor oil and the like are optionally added.
The major component, the hardening agent and the additives are generally mixed with a solvent to prepare the primary primer.
The solvent for the primary primer is a compound which dissolves both the polyamine compound and the polyepoxy compound, and for instance, toluene, xylene, ethylbenzene, methyl ethyl ketone, methyl isobutyl ketone, methyl cellosolve, ethyl cellosolve, butyl cellosolve, an acetate ester of a cellosolve compound, etc. may be used for the purpose.
The equivalent ratio of the polyepoxy compound to the polyamine compound is in a range of 0.5/1 to 1/0.5.
The amount of the silane coupling agent is preferably 0.01 to 20% by weight to the total weight of the polyepoxy compound and the polyamine compound (hereinafter simply referred to as the total weight of the resins). The amount of the pigment is preferably 1 to 500% by weight to the total weight of the resins.
The solvent is used in such amount that the primary primer has a viscosity suitable for coating depending on the required operating conditions. In general, the solvent is used in an amount to make the solution having a concentration of 2 to 90% by weight of the total resins.
As the isocyanate compound having a concentration of isocyanate group of 15-50% by weight, preferably 20-35% by weight which is used as a component of the secondary primer (a concentration of isocyanate means the percentage by weight of isocyanate groups in the isocyanate compound, referred to as NCO% hereinafter), an aliphatic, cycloaliphatic or aromatic isocyanate monomer having at least two NCO groups in its molecule, a mixture thereof or a modified compound thereof is used. Examples of the isocyanate, compound are tolylene diisocyanate (2,4- and/or 2,6-isomers), xylylene diisocyanate, naphthylene diisocyanate, diphenylmethane diisocyanate (MDI), polynuclear polyisocyanate obtained by reacting phosgene with condensation product of aniline and formaldehyde (so-called crude MDI), carbodiimide-modified diphenylmethane diisocyanate and the like. In addition, urethane prepolymer having terminal isocyanate groups which is obtained by reacting the above isocyanate compound with polyhydroxy compound can be used.
The urethane prepolymer includes a reaction mixture of an isocyanate compound and the polyhydroxy compound containing the unreacted isocyanate compound. In this case, the NCO% means the weight per cent of the isocyanate groups in the reaction mixture.
As the polyhydroxy compound, polyether polyol and polyester polyol which are generally used for synthesizing polyurethane are used. For instance, polyalkylene ether polyol of average molecular weight of 300 to 10,000 obtained by adding alkylene oxide such as ethylene oxide, propylene oxide, butylene oxide and the like to polyhydroxy alcohol such as ethylene glycol, propylene glycol, glycerol, pentaerythritol, sorbitol, sucrose and the like; polytetramethylene ether glycol of average molecular weight of 600 to 10,000 obtained by ringopening polymerization of tetrahydrofuran and the like; polyester polyol of average molecular weight of 300 to 6,000 obtained by condensation of bifunctional alcohol such as ethylene glycol, propylene glycol, 1,4-butanediol, hexylene glycol, diethylene glycol, dipropylene glycol and the like with dicarboxylic acid such as adipic acid, succinic acid, azelaic acid, phthalic acid and the like and compounds having terminal hydroxy group and average molecular weight of 300 to 6,000 obtained by ring-opening polymerization of caprolactone are used.
Under normal conditions, isocyanate compounds having NCO% of 15 to 50% are actually usable. When the NCO% is less than 15% strongly adhered composite article can not be obtained. On the other hand, when the NCO% is higher than 50% the adhered surface becomes brittle. The favorable range of the NCO% is 20 to 35%. In addition, in order to accelerate the generation of strong adhesive strength, an aromatic polyisocyanate, for instance, MDI, crude MDI, carbodiimide-modified MDI, or urethane prepolymer thereof is preferable.
In addition, it is preferable to add a high polymeric compound such as chlorinated rubber, chlorinated polyolefin, polyether, polyester or polyurethane, the latter three respectively has weight average molecular weight (determined by gel-permeation chromatography) of more than 10,000 to the secondary primer.
In the present invention, weight average molecular weight is measured by liquid chromatography after calibration by standard polystyrene. A differential refractometer is used as a detector.
The weight average molecular weight is calculated as comparison between the size of polystyrene molecule and that of the high polymeric compound. Weight of each of molecule is calculated on the assumption that a refractive index of each of molecule is same. The calculation is performed by the following equation. ##EQU1## Mw : weight average molecular weight Mi: molecular weight
Wi: weight of molecular weight of Mi (the weight is proportional to height of peak shown in the chart)
By adding such a high polymeric compound as a component to the secondary primer, the coating performance of the secondary primer is improved and the time for exhibiting the adhesive strength is shortened.
Chlorinated rubber is the addition product of chlorine to rubber and which is represented by the molecular formula of(C.sub.5 H.sub.7 Cl.sub.3).sub.x (C.sub.5 H.sub.6 Cl.sub.4).sub.y (C.sub.10 H.sub.11 Cl.sub.4).sub.z and has weight average molecular weight of 5,000 to 100,000 and chlorine content of about 60% by weight. In order to obtain a favorable adhered composite article, it is preferable to use a compound of higher molecular weight.
As the chlorinated polyolefin, chlorinated polyethylene or chlorinated polypropylene which has weight average molecular weight of 5,000 to 60,000 and chlorine content of 60 to 70% by weight may be preferably used. Such a chlorinated polyolefin should be soluble into the solvent, for example, an aromatic hydrocarbon such as toluene or methyl ethyl ketone and the like, to the extent of at least 30% by weight.
As the polyether or polyester, the same kind of polyether polyol and polyester polyol which are used for preparing the afore-mentioned urethane prepolymer and having weight average molecular weight of more than 10,000 are used. As the polyester, commercialized product for instance, PLACCEL.RTM. H-1, PLACCEL H-4 or PLACCEL H-7 (manufactured by DAICEL Ltd.) is used.
As the polyurethane, the same kind of a reaction product of isocyanate compound and polyether polyol or polyester polyol, which is produced for the afore-mentioned urethane prepolymer and has weight average molecular weight of more than 10,000 is used. The polyurethane may have either the terminal isocyanate group or the terminal hydroxy group, however, it is preferable to use the polyurethane having the terminal hydroxy group for the reason of can stability. These high polymeric compound are used in an amount of 0.05 to 3, preferably 0.1 to 2 parts by weight to one part by weight of the isocyanate compound.
The secondary primer is also used in combination with a solvent.
As the solvent for the secondary primer, a solvent which dissolves the isocyanate compound and the high polymeric compound and is inert to isocyanate groups, for instance, toluene, xylene, methyl ethyl ketone, methylene chloride, cellosolve acetate, methyl isobutyl ketone, trichloroethylene, ethyl acetate, butyl acetate and the like, is used in an amount of 0.4 to 30 times, preferably, 0.5 to 10 times by weight to the total weight of the isocyanate compound and the high polymeric compound.
Further, a polar solvent which strongly swells the urethane prepolymer and polyurethane, for instance, dimethylformamide, dimethylacetamide, dimethylsulfoxide, N-methyl-pyrrolidone and the like is preferably used for improving the adhesion of the primary primer and polyurethane elastomer. Such a polar solvent is used in amount of 1 to 100% by weight, preferably 3 to 70% by weight to the total weight of resin components.
By using the primary primer and the secondary primer of the above-mentioned compositions, the polyurethane elastomer will firmly adhere to the metal at a low temperature and to obtain adhered composite article which has excellent water resistance and heat resistance.
In addition, in order to obtain adhered composite article in which the polyurethane elastomer and the metal are strongly adhered together, it is preferable that either or both of the conditions of (a) the primary primer contains the silane coupling agent and (b) the second primer contains the high polymeric compound are satisfied. However in the case where the primary primer is used in such an amount that the dried film thereof has a thickness, which will be described later, of 10-50 microns, a sufficient adhesive strength can be obtained without using both the silane coupling agent and the high polymeric compound.
The polyurethane elastomer used in the present invention is prepared generally by mixing a solution (the first component) of the prepolymer containing isocyanate groups obtained by the reaction of polyisocyanate compound with polyoxyalkylene polyol or polyester polyol and a solution (the second component) containing polyoxyalkylene polyol(or polyester polyol), a chain extender, a catalyst, etc. and casting the mixture (this mixture of the first component and the second component is hereinafter referred to as the raw material for polyurethane elastomer).
Polyoxyalkylene polyol used in the first and second component is preferably the compound obtained by adding ethylene oxide, propylene oxide, butylene oxide and the like to ethylene glycol, propylene glycol, 1,4-butanediol, glycerol, trimethylolpropane, and the like, or the compound obtained by ring-opening polymerization of tetrahydrofuran, and those having average molecular weight of 700 to 10,000 are suitably used.
As the polyester polyol used in the first and second component, the compound having terminal hydroxy group and average molecular weight of 700 to 6,000 obtained by condensation of bifunctional alcohol such as ethylene glycol, propylene glycol, 1,4-butanediol, hexylene glycol, diethylene glycol, dipropylene glycol and the like with dicarboxylic acid such as adipic acid, succinic acid, azelaic acid, phthalic acid and the like and the compound having terminal hydroxy group and average molecular weight of 700 to 6,000 obtained by ring-opening polymerization of caprolactone are preferably used. The bifunctional alcohol and dicarboxylic acid may be used singly or in combination. Also the polyoxyalkylene polyol and polyester polyol may be used singly or in combination.
As the polyisocyanate compound used as the starting material of the prepolymer in the first component, diisocyanate compound such as tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, naphthylene diisocyanate, partially carbodiimidemodified 4,4'-diphenylmethane diisocyanate and the like are preferable. Isocyanate concentration of the prepolymer is 2 to 25% by weight, preferably 2 to 15% by weight, from the viewpoint of reaction rate and properties of the cast product.
As the chain extender, short chain diol such as ethylene glycol, 1,4-butanediol, dipropylene glycol, hexylene glycol and the like, and aromatic diamine such as methylene-bis (o-chloroaniline), methylenedianiline and the like may be used.
The catalyst contained in the second component is those which are generally used for acceleration of urethaneformation, for example, are tertiary amine catalyst such as diazabicyclooctane and the like, and organometallic compound such as stannous octoate, dibutyltin dioctoate, dibutyltin dilaurate, lead octoate and the like.
The metal to which polyurethane elastomer is adhered is iron, aluminum, copper, zinc or alloys containing these metals.
The process of this invention is especially suitable for adhering polyurethane elastomer to the metal at a temperature of lower than 80.degree. C.
Adhesion of polyurethane elastomer to the metal is carried out as follows:
The solution of the primary primer is coated on the surface of the metal so that the thickness of the dried film of the primary primer is 5 to 200 microns, preferably 10 to 150 microns and the coated metal is allowed to stand for more than 2 hours at 20.degree. C. or for more than 30 min. at about 60.degree. C. The water resistance of the adhered article is more improved by heating at a higher temperature. After confirming that the surface of the coated metal has come to be tack-free (non-tacky), the solution of the secondary primer is coated on the surface of the coated metal so that the thickness of the dried film of the secondary primer is less than 200 microns, preferably 10 to 150 microns, and the thus double coated metal is allowed to stand until the solvent has evaporated off, generally, for less than 3 hours. Then the raw material for polyurethane elastomer is poured onto the coated metal. After leaving the thus cast material for about a week at an ambient temperature, the strength of the polyurethane elastomer has been stabilized with the sufficient strength of adhesion. In the case where a strongly adhered composite article is required, the cast material can be heated at a temperature of higher than 100.degree. C. after pouring raw material for polyurethane elastomer on the double coated metal.
As has been precisely described as above, according to the process of the present invention, even though the raw material for polyurethane elastomer which is hardened at a relatively lower temperature is used, a product having favorable adhesion is available at a low temperature without heating after casting. Furthermore, the water resistance and heat resistance of the thus prepared product is excellent.